This invention relates to the field of gantry equipment. More specifically, this invention relates to the gantry drive mechanism for the movement of a gantry crane along a track.
Gantries mounted on wheel truck assemblies that roll along rails are well known in the art of materials handling. In a typical outdoor setting, a trolley is suspended from a gantry that runs along a linear horizontal track system. The gantry is supported above the ground on a truck assembly that runs on rails in a direction transverse to the motion of the crane trolley. The result is that, by a combination of the two mutually perpendicular motions, a large work area of ground may be covered for the movement of materials. The motion of the trolley on the gantry girder is limited by the length of the girder, but the motion of the gantry may be almost unlimited length because of the ability to set runway rail over a large distance.
Gantry cranes have proven to be particularly useful in the forest industry for the movement of logs at mill operations.
Due to the distance between the rails, force of the gantry wheels upon the rails, ground movements due to weather and other forces, the rails on which the gantry runs are not always maintained in perfect parallel alignment. In addition, the carrying of large masses on the trolley at different positions along the gantry girder can result in offset forces that increase the likelihood of misalignment between the runway rails and the gantry wheels.
Wheels for gantry movement are conventionally designed with a double flange to maintain the gantry on the rail should misalignment occur. Misalignment of the wheel and rail results in the side face of the rail contacting the inside flange face of the gantry wheel. This contact results in wear on the flange face which eventually requires expensive wheel replacement.
To extend the operating life of gantry wheels, fixed side rollers have been employed to contact the side of the rail during periods of misalignment and to absorb transverse forces that would be absorbed by flange absent the use of a side roller. Side rollers either could replace or supplement the gantry wheel flange. Wear on a side roller is generally preferred due to the lower cost and the easier replacement of a side roller. Side rollers can be replaced without removing the gantry wheels from the track and therefore result in decreased down time for the gantry. Without the employment of a side roller, the forces of misalignment must be absorbed by the flange of the main gantry wheel contacting the side of the rail. These fixed side rollers have been employed with little or no success. Stresses caused by the runway rail misalignment, imperfections in the various components and dynamic forces result in large unpredictable stress spikes which break the roller assembly. While stresses have less catastrophic impact on the rotating treads of the wheels, the flange to rail contact will act, over time to grind and wear the flanges down by non-rolling metal-to-metal contact, thus requiring a costly premature gantry wheel change. The more out of alignment the system becomes, the faster the wear on the flange and the sooner or more frequent the costly gantry wheel replacement must be undertaken.
An object of the invention is to provide a practical side roller assembly for absorbing forces as a result of rail and gantry wheel misalignment, reduce flange wear and thereby increase wheel life.
Another object of the invention is to provide a system able to indicate to the crane operator misalignment between the wheels and rail.
Yet another object of the invention is to provide a side roller that is of lower cost and simpler to replace than a gantry wheel.
A further object of the invention is to provide a system of side rollers that withstands unpredictable peak stresses without failure.
These and other objects of the invention are accomplished by the following disclosed invention.